A series of studies is proposed to investigate changes related to the development of passive avoidance learning. Young animals of several species have been observed to have difficulty in learning to withhold a response that is followed by punishment. Behavioral techniques will be used to determine whether this deficit is due to deficiencies in responsse inhibition or dificiencies in learning to associate cues with fear. The hippocampal formation has been shown to be important in the learning of passive avoidance tasks. Research is planned to relate the morphological development of the brain to be appearance of effective passive avoidance learning. Because postnatal neuronal proliferation in the hippocampus resents an opportunity for changes in brain structure to occur as a result of experience, in investigation of possibility that there is a critical period for the development of passive avoidance learning will be made. Finally, the generality of the passive avoidance deficit in young animals will be assessed using the flavor toxicosis paradigm.